Based Thermally Activated Delayed Fluorescence Reaching an

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Design strategy for Ag(I)-based thermally activated delayed fluorescence reaching an efficiency breakthrough Marsel Z. Shafikov, Alfiya F. Suleymanova, Rafal Czerwieniec, and Hartmut Yersin Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 19 Jan 2017 Downloaded from http://pubs.acs.org on January 20, 2017

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Chemistry of Materials

Design strategy for Ag(I)-based thermally activated delayed fluorescence reaching an efficiency breakthrough Marsel Z. Shafikov†‡, Alfiya F. Suleymanova†§, Rafał Czerwieniec†*, and Hartmut Yersin†* †

Institut

für

Physikalische

und

Theoretische

Chemie,

Universität

Regensburg,

Universitätsstrasse 31, D-93053 Regensburg, Germany ‡

Ural Federal University, Mira 19, Ekaterinburg, 620002, Russia

§

I. Postovsky Institute of Organic Synthesis, Ekaterinburg, 620041, Russia.

Abstract A design strategy is presented for the development of Ag(I)-based materials for thermally activated delayed fluorescence (TADF). Although Ag(I) complexes usually do not show TADF, the designed material, Ag(dbp)(P2-nCB) (with dbp = 2,9-di-n-butyl1,10-phenanthroline and P2-nCB = nido-carborane-bis-(diphenylphosphine)), shows a TADF efficiency breakthrough exhibiting an emission decay time of τ(TADF) = 1.4 µs at a quantum yield of ΦPL = 100 %. This is a consequence of three optimized parameters: (i) The strongly electron-donating negatively charged P2-nCB ligand destabilizes the 4dorbitals and leads to low lying charge (CT) states of MLL’CT character, with L and L’ being the two different ligands, thus, giving a small energy separation between the lowest singlet S1 and triplet T1 state of ∆E(S1-T1) = 650 cm−1 (80 meV). (ii) The allowedness of the S1→S0 transition is more than one order of magnitude higher than found for other TADF metal complexes, as shown experimentally and by TD-DFT calculations. Both parameters favor short TADF decay time. (iii) The high quantum efficiency is dominantly related to the rigid molecular structure of Ag(dbp)(P2-nCB), resulting from the design strategy of introducing n-butyl substitutions at the 2,9positions of phenanthroline which sterically interact with the phenyl groups of the P2nCB ligand.

In particular, the shortest TADF decay time of τ(TADF) = 1.4 µs at ΦPL =

100 % reported so far suggests the use of this outstanding material for OLEDs. 1 ACS Paragon Plus Environment


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Importantly, the emission of Ag(dbp)(P2-nCB) is not subject to concentration quenching. Therefore, it may be applied even as a 100% emission layer. 1. Introduction Fundamental research focusing on photophysical and chemical properties of organotransition metal complexes is strongly stimulated by potential applications, in particular, in the field of electro-luminescent devices, such as emitters for OLEDs1-15 or LEECs.7,1619

For the emitters, it is crucial that all generated singlet and triplet excitons20 are

harvested and transferred into light. This can be achieved by two different mechanisms: (i) Complexes that exhibit high spin-orbit-coupling (SOC) with respect to the lowest excited triplet state T1, representing triplet emitters, allow to harvest all excitons in the lowest triplet state.21-23 (ii) Compounds that show thermally activated delayed fluorescence (TADF) harvest also all excitons, but the emission occurs essentially via the thermally activated singlet state S1.24-28 Development of TADF compounds is currently under heavy research, since such materials

can be realized with low-cost

and

environmentally friendly Cu(I)

complexes9,24,25,29-44 as well as with purely organic molecules.26,45,46 For organotransition metal compounds, the occurrence of TADF is crucially related to metal-toligand charge transfer (MLCT) states having frontier orbitals, HOMO and LUMO, that are spatially well separated. This leads to a small exchange interaction47,48 between the involved electrons and hence, to a small splitting ∆E(S1-T1) between the lowest excited singlet state S1 and triplet state T1. A small ∆E(S1-T1) value is a necessary condition for obtaining a short (radiative) TADF decay time, being important for minimizing roll-off effects and stability problems in OLEDs. Additionally, the allowedness of the transition from the S1 state to the ground state (S0), S1→S0, that is thermally activated from the lower lying triplet state plays a crucial role. However, basic quantum chemical considerations suggest that the corresponding rate k(S1→S0) and ∆E(S1-T1) are 2 ACS Paragon Plus Environment

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correlated. Small splitting ∆E(S1-T1) requires that the exchange interaction between the unpaired electrons is small. For this, small overlap of HOMO and LUMO is advantageous. At the same time, small HOMO-LUMO overlap leads to a small oscillator strength of the S1→S0 transition, and thus, to small k(S1→S0) and long fluorescence decay time. Indeed, experimental studies on Cu(I) complexes showing TADF reveal that such a correlation exists for a large number of compounds.34 Thus, engineering of TADF materials with shorter TADF decay times is a challenge. For instance, TADF decay times of less than a few µs have not been reported so far29,32,36,39,49 In particular, there are no reports on (radiative) decay times shorter than 3 µs.34,49,50 In this investigation, we focus on a new Ag(I) complex that opens a breakthrough in designing a TADF material with a significantly shorter radiative TADF decay time. In contrast to Cu(I) based compounds, reports on Ag(I) based TADF materials are scarce.37,50,51 This is a consequence of the higher oxidation potential of the Ag+ ion compared to the Cu+ ion.52 Accordingly, the 4d-orbitals of Ag(I) complexes mostly lie energetically below ligand-centered (LC) orbitals. This leads to low-lying states of 3LC character.53-56

Therefore, Ag(I) complexes often do not show TADF, but long-lived

phosphorescence and sometimes even slow intersystem crossing (ISC) resulting in dual emission.55 Obviously, designing Ag(I) complexes that exhibit efficient and shortlived TADF represents an optimization challenge. For this aim, it is required to destabilize the energetically deep-lying 4d-orbitals by an organic ligand with good electron-donating properties. Accordingly, we choose a silver complex containing a chelating nido-carborane-bis-(diphenylphosphine) (P2-nCB) ligand57 (structure shown in Scheme 1). The phosphine coordination induces substantial electron-donating character, which is further strongly enhanced by the negative charge of the nidocarborane

moiety.

In

combination

with

the

chromophoric

3 ACS Paragon Plus Environment

2,9-di-n-butyl-1,10-


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phenanthroline (dbp) ligand, we obtain the neutral Ag(dbp)(P2-nCB) complex. Moreover, this compound is relatively rigid due to the rigid nido-carborane cage and, in particular, the sterically demanding 2,9-di-n-butyl substitutions at the phenanthroline ligand. This allows us to expect high emission quantum yield.31 Here we present the synthesis of the complex, characterize it by TD-DFT calculations and by detailed photophysical measurements investigating properties over a wide temperature range (20 K ≤ T ≤ 300 K). In particular, it will be shown that the resulting Ag(dbp)(P2-nCB) complex exhibits 100 % emission quantum yield and an extraordinarily short radiative TADF decay time, being significantly shorter than reported for any other TADF material so far. 2. Synthesis and molecular structure Ag(dbp)(P2-nCB)

was

obtained

by

reacting

equimolar

amounts

of

silver

hexafluorophosphate (AgPF6), ortho-carborane-bis-(diphenylphosphine) (P2-oCB),58 and 2,9-di-n-butyl-1,10-phennanthroline (dbp) in refluxing ethanol, in analogy to literature procedures57,59,60 (Scheme 1). Under these reaction conditions, orthocarborane of the cationic Ag(dbp)(P2-oCB)+ complex (not isolated) undergoes partial deboronation leading to the respective nido-carborane (nCB) anion,59 thus affording the neutral Ag(dbp)(P2-nCB) complex. Synthetic procedure and structural characterization of Ag(dbp)(P2-nCB) are given in the ESI.

Et2O H C n-BuLi

C

C

C

H =B Ph = C6H5

Li

Ph P C Ph

P(Ph2)Cl

C

Li

Ph

P Ph

EtOH AgPF6 dbp −B

N Ph Ph

N Ag

Ph P

P C

C

Ph

P2-oCB

dbp = 2,9-di-n-butyl-1,10-phenanthroline

Scheme 1. Synthetic route to complex Ag(dbp)(P2-nCB) 4 ACS Paragon Plus Environment

Ag(dbp)(P2-nCB)

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X-ray single crystal diffraction study reveals the Ag(dbp)(P2-nCB) molecule (Figure 1) in which the silver ion is chelated by the two ligands, dbp and P2-nCB, in a distorted tetrahedral coordination. The mean bond lengths to the metal center P-Ag and N-Ag of 2.45 Å and 2.32 Å, respectively, are unremarkable compared to other silver complexes52,54,57,61,62 and are larger than in the analogous Cu(I) complexes due to the larger ion size of Ag+ as compared to Cu+. The bite angles P-Ag-P and N-Ag-N of 86° and 73°, respectively, are smaller than in analogous Cu(I) complexes.63 Voids between the phenyl groups of P2-nCB are partly occupied by the n-butyl chains of dbp. Owing to such steric effects, the arrangement of the ligand side groups is expected to rigidify the structure and thus, to restrain excited-state geometry distortions.64-70

Figure 1. Chemical structure and perspective view (OLEX-271 plot with 50% probability thermal ellipsoids) of Ag(dbp)(P2-nCB). Hydrogen atoms are omitted for clarity. For further data see the ESI.

3. Theoretical calculations based on density functional theory (DFT) and time dependent DFT (TD-DFT) methods By use of DFT and TD-DFT calculations, we can obtain an insight into the electronic structure of Ag(dbp)(P2-nCB). The calculations were carried out at the M0672/def2SVP73 level of theory for geometry optimizations and at the M062X72/def2SVP level for timedependent calculations (See the ESI for details). The optimized geometry of the ground 5 ACS Paragon Plus Environment


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sate (S0) is found to be in a good agreement with the experimental X-Ray geometry (Table S1 in the ESI). Calculations performed for both geometries, S0 and T1, reveal that the lowest electronic transitions, corresponding to states S1 and T1, involve electron density shifts from the metal, the phosphorus atoms, and the nido-carboranyl anion of the P2-nCB ligand to the phenanthroline core of the dbp ligand. This charge transfer results in a flattening distortion of the pseudo-tetrahedral ground state geometry as frequently described for Cu(I) complexes.64-70,74,75 Since the orbital origins of the states T1 and S1 are similar from the perspective of the ground state and mainly emission properties are discussed, we focus on the T1 state geometry. In this geometry, the S0→S1 excitation is dominated by the HOMO→LUMO transition (92 %) perturbed by HOMO-1→LUMO character (4 %). (Figure 2 and Table S2 in the ESI) The HOMO is essentially composed of the metal (13 %) and phosphorus orbitals (47 %) (engaged in bonding to Ag), while the LUMO represents a π* orbital of the dbp ligand (Figure 2 and Table S3 in the ESI). The T1 state is derived from almost the same one-electron excitations as the state S1. Thus, the theoretical predictions allow us to assign the two lowest excited states as

1,3

(MLL’CT) states, wherein L and L’ represent P2-nCB and

dbp, respectively. The calculated energy separation ∆E(S1-T1), obtained as the difference of vertical excitation energies amounts to 0.15 eV (≈ 1200 cm−1) and thus is somewhat larger than the experimentally determined activation energy (Section 4).


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Figure 2. Iso-surface contour plots (iso-value = 0.05) of frontier orbitals of Ag(dbp)(P2nCB), calculated at the M062X/Def2-SVP level of theory in the T1 excited state geometry. Hydrogen atoms are omitted for clarity. For further data see the ESI.

The calculations performed for the triplet state optimized geometry reveal further that the S2 and S3 states result mainly from the HOMO→LUMO+1 and HOMO-1→LUMO transitions, respectively. Since S2 carries the same type of metal 4d character as T1 (and as S1) and since S3 is largely of ligand-to-ligand charge transfer character with no significant contributions from the metal, spin-orbit admixtures of these states (S2 and S3) to the T1 state are expected to be small.76-78 The nearest singlet state with metal 4d contribution different from that in the T1 state is S4 (largely resulting from HOMO-2 → LUMO). It lies 1.57 eV above the T1 level. For such a large energy difference, SOC is expected to be very small. (Compare refs. 34,49,76-78) Thus, a very long phosphorescence decay time is expected to occur. This was experimentally observed, as shown in Section 4. Importantly, the calculations predict a relatively high oscillator strength for the S0→S1 transition of ƒ = 0.0536 (Table S1 in the ESI). For comparison, the ƒ values for chargetransfer transitions of Cu(I) complexes are usually more than one order of magnitude 7 ACS Paragon Plus Environment


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smaller.42,49 Indeed, the high allowedness of the S0↔S1 transition correlates well with the very high fluorescence rate as determined experimentally (Section 4).

4. Photophysical characterization of Ag(dbp)(P2-nCB) 4.1 Spectroscopic introduction In Figure 3, we present a spectroscopic introduction to properties of Ag(dbp)(P2-nCB). The absorption spectrum shows strong absorption bands in the UV region with maxima at 232 nm (57100 M−1cm−1) and 273 nm (37700 M−1cm−1) and a weaker band centered at 385 nm (2100 M−1cm−1). The short wavelength transitions are dominated by π-π* transitions within the dbp and P2-nCB ligands, while the long wavelength absorption

is

assigned,

in

analogy

to

mixed-ligand

phenanthroline-Cu(I)

complexes,36,63,79-84 to transitions with distinct charge transfer (CT) character. TD-DFT calculations predict a MLL’CT type of transition (Section 3).

Figure 3. Absorption spectrum of Ag(dbp)(P2-nCB) measured in dichloromethane (DCM) at 300 K (black line) and emission spectra of Ag(dbp)(P2-nCB) shown in 8 ACS Paragon Plus Environment

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colored lines (λexc = 410 nm) measured at different conditions. The DCM solutions were of ≈ 10−5 M concentration. The PMMA film was doped with ≈ 1 wt.% of the complex.

The emission spectra displayed in Figure 3 are broad and unstructured even if cooled to T = 40 K (or 1.6 K, not reproduced). The emission of Ag(dbp)(P2-nCB) doped in PMMA (λmax = 535 nm) and dissolved in DCM (λmax = 585 nm) is red-shifted compared to the emission measured in powder by 9 nm and 59 nm, respectively. This is related to a flattening distortion of Ag(dbp)(P2-nCB) induced by the CT excitation, as predicted by the theoretical calculations presented in Section 3. In particular, the data show that the distortion-related red shift becomes more pronounced with decreasing rigidity of the environment. (The blue shift of 14 nm as observed on cooling is a consequence of the TADF effect and is discussed below.) Moreover, usually such structural changes are connected with an increase of non-radiative decay rates or reductions of the emission quantum yields due to increasing Franck-Condon (FC) factors of higher excited vibrational wavefunctions of the electronic ground state and of energetically lower vibrational wavefunctions of the electronically excited state.48,85 Indeed, this is also observed for Ag(dbp)(P2-nCB), for which the quantum yield decreases (under degassed conditions) from ФPL(powder) = 100% to ФPL(PMMA) = 85 % and to ФPL(DCM) = 3 %. Similar trends have already been reported for Cu(I) compounds24,34, though the decrease of ФPL is usually more distinct than found for Ag(dbp)(P2-nCB). Obviously, the title compound is rather rigid due to its specific molecular structure. The data of the emission peak positions and the quantum yields are summarized in Table 1.

4.2 Thermally activated delayed fluorescence and drastic increase of the emission decay rate For a deeper photophysical characterization of the TADF properties, investigation of the broad emission spectra is not promising. However, the alternative spectroscopic method, based on temperature- and time-resolving measurements can be applied. Presumably this is the only procedure that allows us to obtain detailed insight into electronic structures in such cases. Corresponding studies are carried out for Ag(dbp)(P2-nCB) powders.86



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Figure 4. Left: luminescence decay times (τ) of Ag(dbp)(P2-nCB) powder measured at different temperatures; Right: The luminescence decay times (τ) plotted versus temperature. The values of k(S1→S0) = 5.6107 s−1 (18 ns) and ∆E(S1−T1) = 650 cm−1 result from a fit of eq. 1 to the experimental τ(T) values, with τ(T1) fixed to 1570 µs as determined directly for T < 60 K (plateau).

Figure 4 (left) displays emission decay curves measured at different temperatures. By use of the emission quantum yields and the decay times (Table 1), one can determine that the radiative decay rate kr = ФPL/τ increases from kr(40 K) = 5.5102 s−1 (assuming the same quantum yield as found for T = 77) to kr(300 K) = 7.1105 s−1. This corresponds to an increase by factor of almost 1300. Obviously, such changes have to be related to the involvement of different electronic transitions at low and high temperature, respectively. They correspond to emissions from the T1 and S1 state to the ground state, respectively. Moreover, the decays are mono-exponential. Thus, we can conclude that the ISC time is fast (compare also refs.66,69,87) and that the two states are thermally equilibrated faster than the measured decay times. In this situation, the emission decay time τ(T) of the system of two states can be expressed by:9,25,34,78,88 ∆ ∆

T

(1)

wherein k(T1) = 1/τ(T1) and k(S1) = 1/τ(S1) are the decay rates with the decay times τ(T1) and τ(S1) of the triplet and singlet excited state, respectively, ∆E(S1−T1) is the energy separation between the S1 and T1 state and kB is the Boltzmann constant. In Figure 4 (right), the decay time is plotted versus temperature. At low temperature, 20 K ≤ T ≤ 60 K, a constant value of 1570 µs is observed (plateau). It represents the phosphorescence decay time τ(T1) for the T1→S0 transition. Phosphorescence decay 10 ACS Paragon Plus Environment

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times of the order of milliseconds were found also for other Ag(I) and Cu(I) compounds.29,34,49,51,74 This long decay displays the spin-forbiddenness of this transition. Obviously, SOC to adequate singlet states is weak. According to the discussion presented in Section 3, the next singlet state (S4) that exhibits a different 4d orbital character than the T1 state and that can induce SOC (following general quantum mechanical rules76-78) is energetically far, with ∆E(S4-T1) = 1.57 eV. Hence, the singlet character mixed into the T1 state is very small. For T > 60 K, the decay time becomes shorter with increasing temperature due to a thermal population of the higher lying S1 state. A plot of the τ values versus temperature has a characteristic form of an s-shaped curve with the point of maximum slope at T = 90 K (τ = 870 µs) (Figure 4). For T > 200 K, the τ(T) values decrease only slightly and reach at ambient temperature τ(300 K) = 1.4 µs. The τ(T) data can be perfectly fitted to eq. (1) (Figure 4) giving the activation energy of ∆E(S1-T1) = 650 cm−1 and the radiative rate of the prompt fluorescence of kr(S1→S0) = 5.6107 s−1 formally corresponding to the prompt fluorescence decay time of τ(S1) = 18 ns. It is remarked that the related prompt fluorescence is not directly observed, since the ISC processes from S1 to T1 are about three orders of magnitude faster (Compare refs.66,69,87). The experimental value of k(S1→S0) = 5.6107 s−1 found for the prompt fluorescence rate is remarkably large. Cu(I) complexes investigated so far (and that have comparable ∆E(S1-T1) splittings) exhibit only rates being more than one order of magnitude smaller.34 Thus, the large (prompt) fluorescence rate is identified as the key factor leading to the exceptionally fast TADF decay time of Ag(dbp)(P2-nCB). This behavior fits perfectly to the large oscillator strength that is calculated for the S1→S0 transition (Section 3). The theoretical value of oscillator strength is also more than one order of magnitude larger than for any other TADF complex reported so far (Section 3).42,49 11 ACS Paragon Plus Environment


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Table 1 summarizes the emission data that characterize Ag(dbp)(P2-nCB). Due to the extraordinarily large radiative TADF decay rate of kr(300 K) = 7.1105 s−1, the emission quantum yield reaches ФPL(TADF) = 100 %. Interestingly at T = 77 K, the quantum yield amounts only to ФPL(77 K) = 87 %, since at that temperature the decay time is as long as τ(77 K) = 1300 µs giving a radiative rate of kr(77 K) = 6.7102 s−1. According to knr = (1-ФPL)/τ, the non-radiative rate is determined to knr = 1102 s−1. Thus, at T = 77 K the non-radiative process can moderately compete with the radiative process, but not at ambient temperature. Table 1. Emission data of Ag(dbp)(P2-nCB) in different environments. powder PMMA CH2Cl2 λmax (300 K)

526 nm

535

585

ФPL (300 K)

100 %

85 %

3%

τ(300 K)

1.4 µs

kr(300 K)

7.1105 s−1

ФPL (77 K)

87 %

τ(77 K)

1300 µs

kr(77 K)

6.7102 s−1

knr(77 K)

1102 s−1

τ(T1, 40 K)

1570 µs

kr(S1→S0)α

5.6107 s−1 (18 ns) 650 cm−1

∆E(S1→T1)α α

determined from the fit of experimental luminescence decay times according to eq. 1, measured for a powder sample of Ag(dbp)(P2-nCB) at different temperatures.

4.3 TADF highlights


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In Figure 5, we highlight the Ag(dbp)(P2-nCB) material´s TADF properties. At low temperature (T < 60 K), one observes only long-living phosphorescence as T1→S0 transition decaying with τ = 1570 µs. With temperature increase (kBT), up-ISC (or reverse ISC, RISC) leads to a population of the S1 state that lies higher in energy than T1. Accordingly, a blue shifted TADF emission of 14 nm (≈ 500 cm−1) is found (Figure 3). This value corresponds well to the activation energy of ∆E(S1-T1) = 650 cm−1 resulting from the decay times’ analysis (Figure 4). Since, the S1→S0 transition rate is much higher than of any other organo-metallic TADF material, the TADF decay time drops to the extremely small value of τ(TADF, 300 K) = 1.4 µs. As a consequence of the related high radiative TADF decay rate, the emission quantum yield reaches 100% at ambient temperature.

Figure 5. Photophysical properties of Ag(dbp)(P2-nCB) as a powder sample shown on a simplified energy diagram. Sometimes up-ISC is also called reverse ISC (RISC).

5. Summarizing conclusions Thermally activated delayed fluorescence is known at least since the pioneering work of Parker et al.89 But only recently, a deeper scientific interest was stimulated by applicability of TADF materials as emitters in OLEDs, as proposed first in the patent literature.27 In particular, it became a challenge to develop efficient TADF materials based on purely organic molecules26,45,46 as well as on organo-transition metals 13 ACS Paragon Plus Environment


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complexes. Within the manifold of transition metal compounds, so far mainly Cu(I)33,34 complexes with distinct MLCT character in the lowest excited states were subject of detailed investigations. Only a few TADF reports exist for Pd(0),90 Pt(0),90,91 and Ag(I)37,50,51 based compounds. In all cases, low-lying charge-transfer (CT) transitions result in relatively small exchange splittings and hence, relatively small singlet-triplet energy separations ∆E(S1-T1) as required for efficient TADF materials. It is a challenge to design Ag(I) complexes with low-lying CT-type excited states that involve significant metal 4d-character, since most compounds exhibit low-lying

1,3

ππ*

states (that do not show TADF).53 This problem could be resolved by designing a Ag(I) complex with a strongly electron-donating negatively charged diphosphine-nidocarborane (P2-nCB) ligand. For TADF materials, in particular, if applied in OLEDs, it is a further challenge to minimize the TADF decay time (at high emission quantum yield). Eq. (1) shows that three parameters govern this decay time: (i) ∆E(S1-T1) should be small, as is frequently discussed.26,33,34,37,43,46,49,50,92,93 (ii) The phosphorescence decay rate k(T1→S0) should be high, which can be obtained for compounds that show high SOC with respect to the lowest triplet state.30,32-34 (iii) The allowedness of the S1↔S0 transition, described by the rate of the prompt fluorescence k(S1→S0) or the corresponding oscillator strength should be high, however, without increasing ∆E(S1-T1). For the first time this problem is addressed and solved. For the title compound, experimental as well as TD-DFT calculations reveal that the radiative rate kr(S1→S0) (or the oscillator strength) are more than one order of magnitude higher than found for any other organo-metallic TADF material with similar ∆E(S1-T1) splitting. In conclusion, the high S1→S0 allowedness found for the title compound leads to the attractive emission properties of Ag(dbp)(P2-nCB), exhibiting a quantum yield of ФPL = 100 % at a decay time of τ(TADF) = 1.4 µs. This represents the shortest radiative 14 ACS Paragon Plus Environment

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decay time of any TADF material reported so far. Moreover, this is the first TADF material with a radiative decay time being comparable to Ir(III) complexes78,94 applied in OLEDs. ASSOCIATED CONTENT *Supporting Information The Supporting Information is available free of charge on the ACS Publications website DOI: Experimental details, synthetic procedures, compound characterizations of the synthesized

ligand

and

complex,

photophysical

instrumentation,

X-ray

data,

computational details, calculated geometries in XYZ format, excited-state energies, dominant orbital excitations from TD-DFT calculations, contour plots of the molecular orbitals that participate in the formation of the discussed excited states, calculated molecular orbital compositions (PDF) (TXT) (CIF) Acknowledgements The authors thank the German Ministry of Education and Research for financial support in the scope of the cyCESH project (FKN 13N12668). MZS gratefully acknowledges Professor Duncan Bruce (York) and The University of York for providing computational facilities. RC thanks the European Research Council (ERC) for support in the framework of the MSCA RISE Project no. 645628. AS acknowledges the German Academic Exchange Service (DAAD) for support. References (1) Yersin, H., Highly Efficient OLEDs with Phosphorescent Materials. Wiley-VCH: Weinheim, 2008. (2) Brütting, W.; Adachi, C., Physics of Organic Semiconductors. Wiley-VCH: Weinheim, 2012. (3) Kim, Y.; Park, S.; Lee, Y. H.; Jung, J.; Yoo, S.; Lee, M. H. Homoleptic Tris-Cyclometalated Iridium Complexes with Substituted o-Carboranes: Green Phosphorescent Emitters for Highly Efficient Solution-Processed Organic Light-Emitting Diodes. Inorg. Chem. 2016, 55, 909-917. 15 ACS Paragon Plus Environment


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(81) Liang, D.; Chen, X.-L.; Liao, J.-Z.; Hu, J.-Y.; Jia, J.-H.; Lu, C.-Z. Highly Efficient Cuprous Complexes with Thermally Activated Delayed Fluorescence for Solution-Processed Organic Light-Emitting Devices. Inorg. Chem. 2016, 55, 7467-7475. (82) McCusker, C. E.; Castellano, F. N. Design of a Long-Lifetime, Earth-Abundant, Aqueous Compatible Cu(I) Photosensitizer Using Cooperative Steric Effects. Inorg. Chem. 2013, 52, 8114-8120. (83) Wang, B.; Shelar, D. P.; Han, X.-Z.; Li, T.-T.; Guan, X.; Lu, W.; Liu, K.; Chen, Y.; Fu, W.-F.; Che, C.-M. Long-Lived Excited States of Zwitterionic Copper(I) Complexes for Photoinduced Cross-Dehydrogenative Coupling Reactions. Chem. Eur. J. 2015, 21, 1184-1190. (84) Wang, Z.; Zheng, C.; Wang, W.; Xu, C.; Ji, B.; Zhang, X. Synthesis, Structure, and Photophysical Properties of Two Four-Coordinate Cu(I)-NHC Complexes with Efficient Delayed Fluorescence. Inorg. Chem. 2016, 55, 2157-2164. (85) Turro, N. J., Modern Molecular Photochemistry. University Science Books: 1991. (86) For most compounds, the decay behavior measured of powder materials is modified, for example, by processes of energy transfer or triplet-triplet annihilation. However, in analogy to investigations of Cu(I) TADF materials [9, 24, 29, 34], also the low-lying CT states of Ag(dbp)(P2-nCB) exhibit geometry distortions even in the relatively rigid crystalline environment. These induce a localization (self-trapping). Thus, the emission of the powder material displays largely molecular properties. Accordingly, concentration quenching does not occur and the decay behavior does not show any distinctive features with concentration increase. (87) Bergmann, L.; Hedley, G. J.; Baumann, T.; Bräse, S.; Samuel, I. D. Direct observation of intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid state. Sci. Adv. 2016, 2, e1500889. (88) Azumi, T.; O'Donnell, C. M.; McGlynn, S. P. On the multiplicity of the phosphorescent state of organic molecules. J. Chem. Phys. 1966, 45, 2735-2742. (89) Parker, C. A.; Hatchard, C. G. Triplet-singlet emission in fluid solutions. Phosphorescence of eosin. T. Faraday Soc. 1961, 57, 1894-1904. (90) Tsubomura, T.; Ito, Y.; Inoue, S.; Tanaka, Y.; Matsumoto, K.; Tsukuda, T. Strongly luminescent palladium(0) and platinum(0) diphosphine complexes. Inorg. Chem. 2008, 47, 481486. (91) Abedin-Siddique, Z.; Ohno, T.; Nozaki, K.; Tsubomura, T. Intense Fluorescence of Metalto-Ligand Charge Transfer in [Pt(0)(binap)2] [binap = 2,2‘-Bis(diphenylphosphino)-1,1‘binaphthyl]. Inorg. Chem. 2004, 43, 663-673.



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(92) Hashimoto, M.; Igawa, S.; Yashima, M.; Kawata, I.; Hoshino, M.; Osawa, M. Highly Efficient Green Organic Light-Emitting Diodes Containing Luminescent Three-Coordinate Copper(I) Complexes. J. Am. Chem. Soc. 2011, 133, 10348-10351. (93) Endo, A.; Sato, K.; Yoshimura, K.; Kai, T.; Kawada, A.; Miyazaki, H.; Adachi, C. Efficient up-conversion of triplet excitons into a singlet state and its application for organic light emitting diodes. Appl. Phys. Lett. 2011, 98, 083302. (94) Hofbeck, T.; Yersin, H. The Triplet State of fac-Ir(ppy)3. Inorg. Chem. 2010, 49, 92909299.

(TOC) It is shown, how to design a new Ag(I) complex that exhibits 100 % emission quantum yield at the shortest TADF decay time reported so far. This is a consequence of (i) a small singlet-triplet splitting, (ii) a rigid molecular structure, and most importantly (iii) a very high S1→S0 transition rate. (Graphical Abstract)


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Based Thermally Activated Delayed Fluorescence Reaching an

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